I'm having this problem as well. It's very annoying. It won't go away unless I quit opera. For now I have to manually open a new window and copy the URL to the new window instead of dragging the tab out.
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RE: [Solved]Tab preview does not clearOpera for Mac
RE: [Solved]Minimized Opera won't restoreOpera for Mac
I'm having this problem as well. It's not a breaking issue, but it's certainly annoying. The top-bar shows Opera so I just open a new window (command + N). It would be nicer to have this issue fixed though.